Whether you drive gas, electric or anything else, there are dozens of different factors that affect your car's efficiency.
That's why, even though Tesla Motors claims a range of 265 miles for its 85 kWh Model S, we've been eager to see how far it can go in realistic day-to-day driving.
Thanks to Motor Trend, we now know. The magazine borrowed Tesla CEO Elon Musk's own Model S and ran it through a series of tests, culminating in a drive down I-15 and then up the Pacific Coast Highway to measure its range.
Starting with a full charge from the California Speedway, where the team logged performance figures, the car was driven down to San Diego on I-15 in 55-69mph traffic, before heading North again using I-5, where much of the driving was stop-and-go.
The rest of the journey North to Redondo Beach was on the PCH, with plenty of stops at intersections. Overall distance? 240 miles.
With a brief stop to top-up within a few miles of their destination, Motor Trend calculated a total range of 238 miles.
That's 11 percent short of the claimed 265 miles, and they note that the trip involved fairly careful driving too. A failure, then?
Not necessarily. While the quoted range wasn't reached, the driver did point out that the trip involved five hours of continuous driving. That's the sort of distance and time that few drivers would go without needing to stop anyway, regardless of how far their battery or gas tank can take them.
The other figures are equally impressive. The drive depleted 93 percent of the battery's capacity, or 78.2 kWh of electricity. Using a gasoline energy equivalency of 2.32 gallons, the car achieved 100.7 mpg-equivalent. Their chase car, a BMW 528i, managed 30.1 mpg. And quick though the BMW is, it doesn't have anything like the potential performance of the Model S.
The 265-mile range may be achievable over long stretches at lower speeds, but even for those drivers not reaching the official target, a range of over 200 miles isn't to be sniffed at.
And save for a criticism about under-padded seats, the car attracted plenty of praise in other areas too.
You can read our own thoughts on the Model S by clicking on our first drive report. And you can leave us your thoughts on the Tesla's range in the comments section below.